The rematching of shades of unknown pigmentation is a central problem in all the colouristics areas of a lacquer company. In particular in the automotive coatings sector, there has in recent years been a continuous broadening of the pigment range. Within this, the number of special-effect shades has increased particularly sharply. Stylists increasingly incorporate any combinations of special-effect pigments into shades. In the light of these developments, efficient methods for minimising effort and cost when rematching this class of shade has great economic importance.
Virtually all special-effect pigment types such as, for example, aluminium, interference pigments or liquid crystal pigments are two-dimensional in character, having a lateral extension of the order of 5 to 40 .mu.m and a thickness of less than 5 .mu.m. Pronounced brightness and special colour effects can be obtained only when the particles are in optimal parallel alignment relative to the surface of the lacquer. The platelet orientation is a property of the individual lacquer system in which these pigments are used, and is dependent on the application parameters.
When rematching special-effect shades of unknown pigmentation, a range of peripheral conditions must be met in order to guarantee the high quality which is demanded, for example, in the automotive coatings sector. The use of the same pigment types as are used in the colour sample which is to be rematched is a prerequisite for working out a high-quality rematch. Only when the type and particle size distribution of the special-effect pigments used correspond to the components of the colour sample is precise rematching free of metamerism possible at all. Moreover, the topology of the platelet-like special-effect pigments in the applied lacquer is of equal importance, since it is this which ultimately determines the formation and conspicuousness of the brightness and special colour effects.
The rematching of shades of unknown pigmentation in colouristics laboratories today is underpinned by computer-aided methods of calculating colour formulations. Colour formulation calculation is a tool for analysing the pigmentation of shades with the aid of reflection spectroscopy in the visible region of the spectrum and using a suitable radiation transport model to describe the diffusion of light in particulate media and thus the reflection spectra which are detectable metrologically.
The quality of the calculated formulations is dependent on standardisation of all the components in a mixed lacquer system and on constancy of the application parameters. Any deviations in the pigment topology of the current shade pattern from the pigment topology of the binder system used for the rematch necessarily lead to simulation results of a lower quality (FIGS. 2A to 2C). If the special-effect pigments are orientated differently in the colour sample which is to be rematched and the binder system used for working out the rematch, there are two possibilities: (i) the horizontal alignment of the platelet-like pigment particles in one's own binder system is less good, and (ii) the orientation in one's own binder system is better, than that in the shade which is to be rematched.
In the former case it is not possible using one's own binder system to achieve a precise rematching of the shade to be processed. If orientation of the particles in one's own lacquer system is better, their topology can be disrupted by the addition of colouristically inert fillers/pigments of suitable particle size distribution which are matched to the topology present in the colour sample which is to be rematched. Since, however, these fillers/pigments are without colouristic effect because their refractive index is comparable with the embedding medium, they cannot be treated like a pigment within the context of colorant calibration and included in the colour formulation calculation. In the conventional colour formulation calculation of special-effect shades, therefore, no account can be taken of topology as an influencing factor.